The present invention relates to a method of depositing a decorative wear-resistant coating layer on a substrate, said substrate constituting at least a part of a decorative and/or utilitarian article.
The expressions "decorative metallic coating" and "metal having decorative properties" are commonly used in the art to designate metallic layers having a brilliant or polished appearance and a resistance to tarnishing and corrosion which are particularly appreciated and appropriate for use in decorative applications.
It also relates to decorative or utilitarian articles made by this method, in which esthetic appearance is important.
It is very often required that the surfaces of decorative articles have a golden colour. When these articles are not of solid gold, but are fabricated from a non-noble metal such as brass, stainless steel, zinc, etc. one may obtain this golden appearance by applying a surface layer of gold or a gold alloy, most frequently by electroplating. If it is desired that this coating be resistant to wear and to corrosion, its thickness must at least attain 10 micron.
To this end, an undercoating is generally electroplated, which is formed of a 14 to 18 carat precious metal alloy. But the corrosion resistance of these alloys is often insufficient, and their colour does not correspond exactly to the colours of solid alloys, such as those defined for example by the standards of the Swiss Watchmaking Industry NIHS 03-50 (1N14, 2N18, 3N, 4N, 5N alloy).
The corrosion resistance of gold platings, and also their colour, may be improved by electroplating a surface layer of gold alloy having a purity higher than or equal to 22 carats, and corresponding exactly to the desired colour.
Given the high price of gold, and its low resistance to wear, it has been sought to replace gold platings by hard coatings deposited under vacuum, or by vapour phase deposition. For example, titanium nitride coatings are generally applied, which are deposited by chemical vapour phase reaction, reactive evaporation, ion projection or cathodic sputtering, on decorative articles of metal, sintered metal carbides or nitrides, or ceramic material. These coatings have the advantage of being resistant to wear and having a golden appearance.
However, the colour obtained with these methods is only approximately that of gold, and a trained eye easily detects the difference. This lack of equivalence will be pointed out hereinafter with reference to FIGS. 2 to 5.
On the other hand, obtaining very dense and corrosion-resistant titanium nitride coatings by ion projection or cathodic sputtering, entails very high compression stress states in the layer, and consequently shear stresses between the layer and the base material which favour separation of the coating.
For an antiwear application, it is proposed in U.S. Pat. No. 3,857,682 to deposit under vacuum a fine gold layer on top of titanium nitride. This idea has been taken up in U.S. Pat. No. 4,252,862 and Swiss Pat. No. 631,040, applied to the field of decoration, with the object of giving the titanium nitride surface the exact colour of gold, or of a gold alloy. During utilization of an article thus coated, the wear of the gold coating occurs only at the sharp edged angles and makes apparent the colour of titanium nitride, whose colour is slightly distinguishable from the remainder of the coating.
To improve the brillance and colour conformity of titanium nitride coatings, Japanese publication No. 58.153.766 and European publication No. 38.294, describe a method of conjugated deposition of titanium nitride and gold, for forming on the whole or a part of the coating thickness a titanium nitride/gold compound. This procedure nevertheless seems to pose corrosion problems, and the colour obtained is also away from the standard colours of golden coatings.
Finally, the successive deposition of thin layers of titanium nitride and gold, by a vacuum process, also improves the brilliance of the coating.
All of these known methods unfortunately have as principal defects:
The risk of coating separation induced by shear stresses at the surface of contact of the titanium nitride and the base material.
Random, often poor adherence of gold on titanium nitride, except in the case of simultaneous deposition of titanium nitride and gold.
The difficulty of obtaining a standard colour by vacuum deposition, and especially of varying the colour as a function of the utilizers' demands, while the methods of depositing gold or gold alloy do not enable varying the final colour of the coating from one treatment to the other.